This invention relates to a method and apparatus for controlling and conserving energy utilized in an absorption refrigeration system. More specifically this invention relates to modulating heat input to an absorption concentrator by generally maintaining a set return chill water temperature.
Absorption refrigeration systems are typically used to cool large office buildings and the like. More specifically system chilled water is cooled by a central absorption generator unit. The chilled water is then circulated throughout a building and fed through cooling coil units mounted periodically in zone air ducting. Warm room air is drawn into the air ducting through a filter and the cooling coil where heat from the air is absorbed and transferred to the circulating chilled water. The cooled air is then blown back into the occupant zone to maintain a desired temperature level. The warmed system chilled water is returned to the absorption generator unit where heat is removed and transferred to a cooling tower and the chilled water is returned to the system cooling coils in a continuous closed loop process.
Absorption generator units typically comprise an enclosed cylindrical shell, which enhouses, in a closed refrigerant loop, a concentrator, a condenser, an evaporator and an absorber.
Briefly, an absorption cycle begins with a refrigerant, such as water, held in solution with a salt, such as lithium bromide, in the concentrator. This solution is selectively heated by, for example, piping steam or hot water through the solution. As the solution temperature rises, refrigerant water boils out of solution and is cooled and collected as a liquid in the condenser.
The condenser refrigerant is then fed into a low pressure evaporator chamber where the refrigerant water evaporates and absorbs heat from the surrounding environment. In this connection warm system chilled water returning from a cooling load is piped into the evaporator section through a heat exchange tubular bundle and back out for circulation to the external load in a closed loop. Heat is absorbed from the system chilled water in the evaporator in accordance with the latent heat of vaporization of a water refrigerant of approximately 1000 BTU per pound of refrigerant.
The refrigerant vapor generated in the evaporative cooling process spills downward into the absorber containing a salt solution of lithium bromide. The salt solution absorbs the refrigerant vapor liberating heat energy in the process. This heat is removed by water circulating from a cooling tower and the resultant solution of refrigerant and salt is then pumped into the concentrator and the process is repeated in a continuous closed loop.
Energy to drive the generator is input to the absorption unit via the concentrator and useful cooling work is performed by lowering the system chilled water temperature as it passes through the evaporator. Variation in cooling output can be achieved by regulating the energy input to the concentrator. In this regard if the temperature of the concentrated solution is increased more refrigerant will be liberated to be condensed and delivered to the evaporator.
Control of the absorption generator unit has been achieved in the past by maintaining a set temperature level of system chilled water leaving the evaporator. More specifically, chilled water temperature is monitored as it leaves the evaporator and is compared to a design set point. A valve which regulates steam input to the concentrator is then modulated in accordance with variations in the supply chilled water temperature from the design set point.
As previously indicated, chilled water is circulated through air handling units in a building and heat in the air is removed by a cooling coil supplied with the chilled water. The amount of chilled water that is circulated through the building is the amount that is required at the cooling coil to accomplish the maximum design cooling requirement at each of the air handling units.
When the cooling load is less than design peak (a condition that exists most of the time), the system will be providing more chilled water to the air handling unit than is required to satisfy the cooling load. Each air handling unit, however, is controlled by a thermostat which operates a chilled water bypass valve. In conditions of less than peak loading some of the supply water is diverted directly to the return and only part of the chilled water passes through the cooling coil.
The result of this method of control is to cause the chilled water supply temperature to remain essentially constant and to cause the chilled water return temperature to drop approaching the supply temperature as the cooling load is decreased. The energy effect of this type of control is to cause the energy consumption of the absorption generator unit to drop in direct relation to the machine output capacity, maintaining nearly constant energy consumption per ton of generator unit capacity.
Accordingly, it would be highly desirable to provide a method and apparatus for controlling an absorption generator unit where the machine energy consumption per ton of cooling output could be reduced at partial load conditions.
In addition conventional control systems frequently attempt to minimize energy consumption of an absorption generator by revising the set point of the leaving or supply chilled water temperature in accordance with variations in outside air temperature. Such conventional systems function adequately as long as the actual building load is equal to a design assumed load. However, if the actual building load exceeds the assumed load, then the supply water temperature that results from the reset controller will be too high. If the supply water temperature is too high, then the cooling coil will be unable to remove heat from the air and the actual cooling load of the building will never be transferred to the chilled water loop and, thus, will never be removed by the evaporator.
Since outside temperature typically affects only a small portion of a cooling load (30% in office buildings), the above situation may occur frequently. It would therefore also be desirable to provide a method and apparatus for controlling an absorption generator unit which would be suitable to take advantage of decreases in outside air temperature while concomitantly providing adequate cooling capability for a system load even when the system load exceeds assumed design values.